1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copier, a printer or a facsimile, and relates to a process cartridge used for the image forming apparatus. More specifically, the image forming apparatus of the present invention includes a plurality of image formation units. Each image formation unit includes a latent image support for supporting an electrostatic latent image, and a charging unit for charging the latent image support surface. A latent image formation unit forms an electrostatic latent image on the latent image support. A developing unit develops and forms the electrostatic latent image into a toner image. A transfer unit transfers the toner image on the latent image support to a recording medium. Moreover, the present invention relates to a process cartridge used for the image forming apparatus having the plurality of image formation units each equipped with the recording medium surface side the transfer unit to which the recording medium surface side is moved so that the surface of each latent image support of the image formation units is countered one by one.
2. Description of The Related Art
With the electro-photographic equipment, color printers and color copiers performing color image formation are increasing with the demand from the market today.
Among the color electro-photographic equipment, there is a one-drum type image forming apparatus in which the surroundings of one photoconductor are equipped with the developing units of two or more colors, a toner is adhered with those developing units, a synthetic toner image is formed on the photoconductor, the toner image is transferred to a copy sheet, and it is fixed as a printed color image on the copy sheet. On the other hand, there is a tandem type image forming apparatus in which two or more photoconductors are equipped with individual developing units respectively, a monochrome toner image is formed on each photoconductor respectively, those monochrome toner images are transferred one by one, a synthetic color image is printed on a copy sheet.
A description will now be given of comparison between the one-drum type and the tandem type. In the one-drum type, the number of photoconductors is one, and the miniaturization can be easily attained and the one-drum type has the advantage that the cost reduction is possible. However, in order to repeat the image formation two or more times (usually 4 times) using one photoconductor and to form a full color image, the improvement in the speed of image formation is difficult.
On the other hand, in the case of the tandem type, the scale of the equipment becomes large, which causes a problem that the equipment becomes expensive. However, the tandem type image forming apparatus has an advantage that the improvement in the speed of image formation is easy.
There is an increasing demand for a full color image being formed at a high speed that is the same as the speed of monochrome image formation, and the tandem type has attracted attention.
In the case of the tandem type, four photoconductors are arranged in parallel for each color image formation of cyan, magenta, yellow and black, and the image forming apparatus of the tandem type needs to make smaller the diameter of each of the photoconductors and shorten the distances between the four photoconductors, in order to create more space, reduce the equipment size, and achieve cost saving.
As a transfer method to transfer a toner image from a photoconductor to a recording medium, there are an indirect transfer method in which a bias roller is arranged at a downstream portion of the photoconductor, and a direct impression method in which the pressure of an elastic roller as a bias roller on the photoconductor is directly applied to a recording medium between the elastic roller and the photoconductor.
FIG. 7 shows a direct transfer method in a conventional tandem type image forming apparatus. As shown in FIG. 7, in the direct transfer method, an image on each of photoconductors 1 is transferred to a recording sheet xe2x80x9csxe2x80x9d, which is transported by a sheet transport belt 3, by using a corresponding one of transfer units 2, respectively.
In a conventional indirect transfer method, if the diameter of each of the photoconductors and the distances between the photoconductors are reduced in order for the miniaturization and cost saving, it becomes difficult to control the electric current which flows from the bias roller into the photoconductor.
FIG. 6 shows an indirect transfer method in a conventional tandem type image forming apparatus. In FIG. 6, reference numeral 40BK indicates a photoconductor for a black image, reference numeral 40Y indicates a photoconductor for a yellow image, and reference numeral 10 indicates an intermediate transfer belt which is brought into contact with these photoconductors in common.
As shown in FIG. 6, a bias roller 603BK pushes up the intermediate transfer belt 10 indirectly from the upstream side of the intermediate transfer belt 10 about the photoconductor 40BK in the conveyance direction. The pressure of the belt 10 is applied to the intermediate transfer belt 10 against the photoconductor 40BK. Reference numeral 603BKxe2x80x2 indicates an auxiliary roller, and the auxiliary roller 603BKxe2x80x2 currently pushes up the intermediate transfer belt 10 from the downstream side of the intermediate transfer belt 10 about the photoconductor 40BK.
Similarly, reference numeral 603Y indicates a bias roller for the photoconductor 40Y, and reference numeral 603Yxe2x80x2 indicates an auxiliary roller.
The auxiliary rollers 603BK and 603Y are grounded respectively, and the bias roller 603BK and 603Y are connected to bias power supplies, respectively. For example, if the electric current i2 which flows to the ground side from the bias roller 603BK is larger than the electric current i1 which flows into the photoconductor 40BK, it becomes difficult to keep constant the electric current i1 which flows into the photoconductor 40BK.
The intermediate transfer belt is usually made of a material with middle resistance, and when changes of resistance and electrostatic capacitance according to the environment cause the electric current i2 to fluctuate, it is very difficult to maintain the transfer conditions of the intermediate transfer belt uniformly by keeping the electric current i1 constant.
The ratio of the electric current i2 to the electric current i1 becomes large as the distance L between the photoconductor 40BK and the photoconductor 40Y becomes small. The electric current control will become more difficult in such a case. To avoid this, the bias roller is arranged at the downstream side of the photoconductor in the rotation direction of the intermediate transfer belt 10. Moreover, to avoid the influences on the other photoconductors as much as possible, it is necessary to take a slightly large distance between the photoconductors and more space than the direct transfer method.
For this reason, in order to attain the miniaturization of equipment, in the tandem type image forming apparatus having the four-drum photoconductors and the intermediate transfer mechanism, the direct transfer method is adopted in many cases. By the direct transfer method, the bias roller serves as a configuration which counters a photoconductor from the front through the intermediate transfer belt, and the electric current which flows into the photoconductor decreases very much, and it becomes possible to keep constant the electric current which flows into the photoconductor.
However, the bias roller is pressurized by the photoconductor from the front through the intermediate transfer belt when adopting the direct transfer method, and the pressure acts on the toner on the photoconductor surface greatly. The toner image tends to be condensed, and the toner at a certain spot of the image on the photoconductor remains without transfer. There is a problem that an undesired image which lacks the toner corresponding to the spot is formed on the recording paper.
In order to avoid the problem of the undesired image, the speed difference in the surface speed of the photoconductor and the recording medium is preset according to a conventional method. However, according to this method, the transfer accuracy at the time of transferring of a toner image from the photoconductor to the recording medium will become worse, and dot reproducibility and thin-line reproducibility will degrade.
Furthermore, the tandem type image forming apparatus requires high accuracy with respect to the respective rotation speed of the photoconductors and the transfer speed of the recording medium. This is because irregularity of the rotation of each drive system, the gap of rotating speeds, etc. cause color gap and a banding image immediately. Therefore, although the speed difference has arisen not a little in the surface speed of the photoconductor and the recording medium by these factors, establishing the speed difference intentionally at the surface speed of the photoconductor and the recording medium will cause degradation of the image quality.
Moreover, the surface abrasion loss of the four photoconductors by the repetition image output in the tandem type image forming apparatus is not uniform. According to the kind of toner, the coating weight, the number of times of development, the material of a contact member and the variation of the transfer pressure, etc., the surface abrasion loss is varied and a difference in the circumference of each photoconductor occurs. Therefore, the speed difference of the surface speeds of the photoconductors and the recording medium also becomes large by expansion of the circumference difference.
When the surface frictional coefficient of a photoconductor is small, the contact portion of an electro-photographic photoconductor and a recording medium can cancel the strain of the recording medium by the speed difference with a slight slippage of the recording medium while the strain is small. On the other hand, when the surface frictional coefficient of a photoconductor is large, the contact portion cannot slip easily, and the contact portion slips only after the strain of the recording medium is accumulated and a larger pressure is applied to the recording medium. If the cancellation of such strain due to the slippage becomes severe, it will cause a banding image to appear, and will become a factor of degradation of the image quality.
An object of the present invention is to provide an improved image forming apparatus in which the above-described problems are eliminated.
Another object of the present invention is to provide a tandem type image forming apparatus that can reliably prevent the occurrence of an undesired image, such as color gap and banding, over an extended period of time.
Another object of the present invention is to provide a process cartridge used for a tandem type image forming apparatus which can prevent the occurrence of an undesired image, such as color gap and banding, over an extended period of time.
The above-mentioned objects of the present invention are achieved by an image forming apparatus which image forming apparatus having a plurality of image formation units, each image formation unit comprising: a latent image support supporting an electrostatic latent image; a charging unit charging a surface of the image support; a latent image formation unit forming an electrostatic latent image on the image support surface; a developing unit developing the electrostatic latent image with toner to form a toner image on the image support; and a transfer unit transferring the toner image from the image support to a recording medium, the image forming apparatus comprising a recording medium transnsport unit transporting the recording medium so that the surfaces of the image supports of the plurality of image formation units are countered to a surface of the recording medium, respectively, wherein the surface of each image support has a coefficient of friction that is in a range of 0.1 to 0.7 according to Euler belt method, and a difference between a rotation speed of each image support and a transport speed of the recording medium is below 1%, and each image support has an outermost surface containing at least a filler and a binder resin.
The above-mentioned objects of the present invention are achieved by a process cartridge for use in an image forming apparatus, the image forming apparatus comprising: a main body; a latent image support supporting an electrostatic latent image; a charging unit charging a surface of the image support; a latent image formation unit forming an electrostatic latent image on the image support surface; a developing unit developing the electrostatic latent image with toner to form a toner image on the image support; a transfer unit transferring the toner image from the image support to a recording medium; and a recording medium transnsport unit transporting the recording medium so that the surface of the image support is countered to a surface of the recording medium, wherein the process cartridge includes the image support and at least one of the charging unit, the image formation unit, the developing unit, the transfer unit and an image support cleaning unit, which are integrally provided in the process cartridge, and the process cartridge is provided to be removable from the main body, wherein the surface of the image support has a coefficient of friction that is in a range of 0.1 to 0.7 according to Euler belt method, and a difference between a rotation speed of the image support and a transport speed of the recording medium is below 1%, and the image support has an outermost surface containing at least a filler and a binder resin.
According to the image forming apparatus and the process cartridge of the present invention, the change of the outside diameter of a latent image support can be made small even in the repetition use or over an extended period of time, and it is possible to reliably form a good full color image without outputting a banding image.